A conventional black level detection circuit of a video signal was laid open with the JPO unexamined publication No. H02-12427. FIG. 8 is a block diagram of the conventional black level detection circuit of a video signal.
A comparator 21 outputs a high level signal to a first input terminal of an AND circuit 22 when a sampled value newly fed into the comparator 21 is smaller than the current minimum value of the received video signal. A subtraction circuit 23 calculates a difference between the current minimum value and the next sampled value of the video signal, and an absolute value circuit 25 finds an absolute value of the difference. A comparator 27 compares the absolute value of the difference fed from the absolute value circuit 25 with a quantizing error fed from a level setting circuit 26. When the absolute value of the difference is greater than the quantizing error, the high level signal is output from the comparator 27 to a second input terminal of the AND circuit 22. The output of the comparators 21 and 27 are fed into the AND circuit 22 and when the AND circuit 22 outputs the high level signal, a latch circuit 24 holds the new sampled value and then renews the current minimum value.
When using the above structure, a drift due to the quantizing error can be avoided when measuring the minimum value, however; noises other than the quantizing error have contributed to an error in detecting the black level, e.g. when impulse noise, such as, negative noise spikes, lower than the black level is included in the video signal, the impulse noise is detected as the black level of the video signal by mistake. As a result, when using this erroneous result of the black level detection as an automatic control signal over a brightness and a contrast in displaying a video, the video is inappropriately adjusted apart from an actual image, thereby producing a video uncomfortable to watch.
Further, when using the above conventional structure, minimum values are continually detected, whereby noises outside the video signal period such as a blanking period may be detected. If this result of the black level detection is used as the automatic control signal over the brightness and contrast in displaying the video, it further degrades the picture quality.